4. Michael Richards Apologizes (November 20, 2006)

We've become accustomed to cringe comedy, but Emmy-winning Seinfeld star Michael Richards plunged into ugliness during a failed stand-up appearance in November 2006, which devolved into him hurling racial epithets at audience members. Days later, Jerry Seinfeld appeared on the Late Show, bringing on his longtime friend to apologize via satellite. It was a PR disaster: The inarticulate, flustered Richards rambled, acting not that much different than his daffy Kramer character, which prompted awkward giggles from the crowd. (Among the segment’s many low points is Seinfeld meekly chiding the audience for laughing.) Richards' career never really recovered, and both Letterman and his guest seemed relieved once the disgraced star's spot was over.